1. Field of the Invention
The present invention relates to a vehicle, and in particular to learning control for correcting an error of a resolver which detects a rotation angle of a motor.
2. Description of the Background Art
An electrically powered vehicle such as an electric vehicle and a hybrid vehicle is equipped with a motor which generates a drive force. In order to control the motor with accuracy, it is necessary to detect a rotation angle of the motor. A resolver is a rotation angle sensor which has a very simple structure and enables normal and highly accurate angle detection under a situation in which temperature, vibration, and oil environments are very severe. The resolver is widely used in electrically powered vehicles.
It is known that an electric signal output by a resolver includes an error resulting from a gap between axial centers of a rotor and a stator of the resolver, or the like. Japanese Patent Laying-Open No. 2013-72686 discloses a technique for appropriately correcting an error included in a rotation angle detected using a resolver.
In order to control a motor with accuracy, it is desirable to perform learning control which corrects an error of a resolver and stores the result. In order to perform such learning control of the resolver for the motor, a rotation speed of the motor should be more than or equal to a prescribed speed. When the rotation speed of the motor is more than or equal to the prescribed speed, it can be considered that the rotation angle of the motor increases equally. Thus, if fluctuation occurs in a change amount of the angle detected by the resolver, this can be considered as an error.
Generally, most systems using a motor are not provided with a transmission, because they can output a high torque even when the motor is rotated at a low speed. However, a hybrid vehicle may incorporate an automatic transmission, because it also uses an internal combustion engine which cannot output a high torque at a low rotation speed. Further, even an electric vehicle may incorporate an automatic transmission in order to enhance acceleration performance during high-speed traveling, and the like.
In a vehicle in which an automatic transmission is incorporated into an output shaft of a motor as described above, the automatic transmission can control a rotation speed, and thus the vehicle may be controlled to suppress the rotation speed of the motor to be low. Accordingly, there is a possibility that an opportunity to perform learning control of a resolver cannot be ensured as desired.